There is considerable interest within industry in manufacturing prototypes or small production batches in a rapid and cost, effective manner. Conventional part production methods are neither time nor cost effective when only a small number of units are needed because they require expensive part-specific tooling, setting up machining protocols, and generating and programming three-dimensional (3-D) tool paths which require much time and professional expertise. The cost and lime to set up and run machine-specific tooling, along with the initial capital costs for tooling, make conventional prototyping/small production run processes both time and cost intensive. Furthermore, conventional prototyping methods are limited in practice to simple part geometries. Complex parts involving inner features and cores/cavities are difficult to produce using conventional techniques and often require precision casting methods which are highly expensive, time consuming, and require a broad range of expertise.
Rapid Prototyping Systems (RPS) in the prior art, to a large degree, attempts to address the needs and problems mentioned above. A single automated system can be used to produce prototype parts and small production runs directly from engineering designs. Such a system is limited only by the size of the part and not by the intricacy oil the part geometry. Thus, such a machine is not so part-specific as conventional tooling, and a capital investment in such a machine is all that is needed to produce virtually any part within the size constraints of the system. Automated prototyping machines, furthermore, require a minimum of human expertise for successful operation and a relatively negligible amount of set up time for a particular part. Parts of complex geometries can be realized in relatively short amounts of time with significant benefit to industry especially where designs are changed frequently and prototypes or mock-ups are needed for design evaluation.
All such RPS make use of a common approach involving the stratification of the prototype. Software "slices" the prototype part geometry into a sequence of cross-sectional contours(strata) used to drive a materials processing system which recreates each contour out of prespecified materials. The prototype part is built up by adhering successive part cross-sections together until the part is complete.
Currently, there are several realizations of RPS, each employing a particular technology with its own strengths and weaknesses:
Stereolithography, referenced in U.S. Pat. No. 4,575,330 by Charles W. Hull; PA1 Computer Aided Manufacturing Process and System (CAMPS), as described in U.S. Pat. No. 4,665,492 by William E. Masters; PA1 Laminations Method, described in U.S. Pat. No. 4,752,352 by Michael Feygin; PA1 Selective Laser Sintering (SLS), described in U.S. Pat. No. 4,863,538 by Carl R. Deckard; PA1 Mask and Deposit (MD*) System, described in U.S. Pat. No. 5,126,529 by Lee E. Weiss.
Three Dimensional Printing, discussed in the Publication "Three Dimensional Printing: Form, Materials, and Performance", by Michael J. Cigna and Emanuel M. Sachs, Solid Freeform Fabrication Symposium, University of Texas at Austin, Austin Tex., August 1991.
Stereolithography produces part layers by scanning a laser across the surface of a photopolymer liquid bath and curing the photopolymer along the part contours. The precision is only limited by the boundary of photopolymerization initially, but as the parts continue curing in the post build stage, warpage becomes a limiting factor. The material properties of the parts are also limited by the material properties of photopolymers.
Selective laser Sintering (SLS) produces part layers by scanning powder with a high powered laser to induce sintering local to the laser path. While this process creates complicated geometries, the sintered material densities are low. Consequently, the mechanical properties of SLS parts are relatively unsuitable for functional prototypes. Increasing the density of SLS parts would require a higher degree of sintering/melting of the part powder, thereby compromising the geometric control provided by SLS.
Rapid prototyping by Precision Metal Spraying (Mask and Deposit),on the other hand, employs precision cut masks for each part layer and thus has the advantage of geometric control limited by the mask precision and/or the aspect ratio mask feature width-to-layer depth), notwithstanding any problems associated with mask/part material selection such as binding, warping, and mask melting. The intra/interlayer bonding in sprayed parts, however, is often primarily mechanical because the material particles are cooled before they hit the surface layer. Obtaining stress-free layers with desirable material properties involves significant tradeoffs with geometric control of the part.
Similarly, parts made by Ballistic Particle Manufacturing or InkJet Printing (CAMPS) must also trade off part quality with geometric control. The building of prototypes from welded or adhered precut laminations as in the Laminations Method, moreover, suffers from a tradeoff between inter-lamination bonding and geometric control, as well as from several precision handling problems associated with complicated part cross sections.
Lastly, 3-D printing systems deposit tiny binder material droplets onto a layer of powder, essentially precision printing each successive part layer. These systems, however, have the disadvantage of porosity (low density) and poor bonding in green parts (prior to oven baking) due to the fluid mechanics and physics of the printing and binding processes. Once the parts are oven heated to bake out the binder material, warpage and distortion related to shrinkage limit the attainable precision of the final parts.
All of the present rapid prototyping methods, therefore, unfortunately are subject to contain inherent difficulties and limitations in aspects of their prototype creation. In summary, the key disadvantages associated with one or more of the current systems are: (1) poor material properties and/or distribution of material properties; (2) poor geometric control and/or difficulty with complex geometry; and (3) trade off between geometric part control and interlayer bond strength and/or part properties, such as density or microstructure.
While each of the above prior processes is different, they all share a key element: each process is additive. That is, they all produce a 3-D part by incrementally adding material to built it up. Furthermore, the additive technologies which distinguish these processes all involve a tradeoff between maintaining a high degree of geometric precision and attaining suitable material properties in the final part.
In accordance with the present invention on the other hand, 3-D manufacturing by controlled layerwise deposition/extraction is a novel rapid prototyping method that integrates the precision control of established subtractive processes with additive processes optimized to produce desired material properties in order to produce a superior rapid prototyping system that does not suffer from any of the above mentioned shortcomings of prior art systems.
In the controlled layerwise deposition/extraction method of the invention, each layer is formed by selectively depositing part and complementary materials on the preceding layer (work surface). Complementary material surrounds the part material on each layer to provide such things as structural support, chemical and/or thermal integrity, and adhesion; such being chosen based on the specific materials and the specific implementation of the method. Systems for selective deposition of materials are generally based on the properties of the materials and will be discussed in more detail hereinafter. Part materials are extracted from the initial part contours produced by the deposition systems to arrive within specified geometric tolerance of the part material layer. Selective deposition and extraction of complementary materials cats be used to form control contours for the selective deposition of part materials. These control contours are used to guide the deposition system into producing near net shape part contours such that the amount of subtractive processing for the part material layer is reduced. Precut masks can be used as a form of control contour for selective deposition. By preforming masks from complementary materials and leaving them in place, the additive processing of complementary materials can also be reduced. The type of control contour and the level of precision in any control contour will be determined by the part and complementary materials, their respective area ratios over the work surface, and/or the particular embodiment of the method. In any case, each completed layer is an aggregate of part materials contours and complementary materials contours, the part materials contours being within prespecified geometric and material property tolerances and the remaining area of the aggregate layer (work surface) filled with complementary materials. By employing various deposition and extraction processes, and by maintaining strict control of the processing environment, the method for rapid prototyping of the invention may be used with many different materials including, for example, metals, alloys, thermoforming plastics, thermosetting plastics, ceramics, and salts. These different materials may be used simultaneously within the part.